1. Field of the Invention
The present invention relates to a high performance controller for a 3-phase converter to be used for UPS, VVVF, active filter, high power factor converter, or vector control using a cycloconverter.
2. Description of the Prior Art
FIG. 1 is a block diagram of an inverter controller of the prior art disclosed, for example, in the Intelec Transaction PP. 205-212 (Oct. 18-21, 1983, Tokyo) entitled "Inverter Output Voltage Waveform Closed-Loop Control Technique". In this figure, 1 is an inverter main circuit; 2, 3 are a reactor and a capacitor, respectively, forming an AC filter; 4 is a DC power supply; 5 is a load; 6 is a control circuit of inverter main circuit 1. This control circuit 6 is composed of an AC (sine wave) reference voltage generator 7, a subtractor 8 which subtracts a feedback voltage from the output of the reference voltage generator 7, an amplifier 9 and a pulse width modulation (PWM) circuit 10, which is formed by a comparator 11 and a carrier generator 12. A numeral 13 is a drive circuit which operates the inverter main circuit 1 on the basis of an output of the carrier generator 12. Operations are then explained hereunder.
First, a sinusoidal output voltage is obtained across the terminals of capacitor 3 in compliance with the control output of the PWM circuit 10 (the control system is explained on the image of analog system). Meanwhile, an output of the PWM circuit 10 controls the switching of inverter main circuit 1 through the amplifier 9 so that the sine wave reference of the AC reference voltage generator 7 matches the output voltage thereof. Moreover, the PWM circuit 10 is formed by a triangular carrier generator 12 and a comparator 11 and determines the timing of switching operation of PWM in accordance with the sinusoidal signal obtained by amplifying a voltage deviation in the amplifier 9. In actuality, since the amplifier 9 has only finite gain from the point of view of stability of operation, an output voltage of the inverter follows the reference voltage under the condition that an output voltage of inverter has little deviation from the reference voltage.
Next, a second example of the prior art will be explained with reference to FIG. 2. This figure is the same figure as FIG. 6 on page 54 of a paper entitled "Decoupling Control of Instantaneous Reactive Power Compensation by PWM Power Converter" which was presented on the conference record of the JIEE Technical Meeting of Semiconductor Power Converter issued in 1984 as paper number SPC-84-80. In FIG. 2, for the sake of convenience of explanation, the same graphical symbols as those of other figures of this application are used. The control circuit of FIG. 2 generates the signal which is obtained by amplifying a difference between the current commands Id*, Iq* of the d and q axes and the current feedback signals Id, Iq of the d, q axes with the gain K. The d axis indicates the perpendicular axis element in the current vector while the q axis indicates the lateral axis element.
Moreover, a signal is obtained by multiplying the angular velocity .omega. of output frequency of the converter and output inductance of L.sub.s to the feedback signals Id, Iq of the d, q axes and the current components of d, q axes are prevented from interfering mutually with the other phases, by subtracting the q axis element of above signal from the inverter voltage command of d axis and adding the d axis element of the above signal to the inverter voltage command of q axis.
Since the control circuit of the converter of the prior art is formed as explained above, the inverter operates as a voltage source having very low impedance when it is observed from the output side. Therefore, if short-circuit trouble occurs in the load side of the inverter or if in-rush current of a transformer flows, an excessive output current flows, resulting in a problem that the overcurrent condition is easily generated and thereby protection becomes difficult. In addition, if a load such as a rectifier which generates a lot of harmonics is connected in the output side, the voltage deviation increases as explained previously, and the control operation is carried out after such deviation appears, thereby resulting in a problem that a voltage deviation is inevitably left in compliance with load harmonics.
Moreover, the decoupling method of the 3-phase inverter disclosed in the material of the JIEE Technical Meeting Semiconductor Power Converter SPC-84-80 gives the decoupling method in the control circuit used in the continuous system. The decoupling method in the sample control system of the prior art has been realized under the assumption that the sampling time is sufficiently short. Namely, the long sampling time has brought about a problem that decoupling control cannot be performed completely.